Melbourne17.2 River outflow from the region

Supporting Information

The volume of 2,502,127 ML recognised in the water accounting statements represents the volume of water discharged from rivers and streams in the Melbourne region to Port Phillip Bay and Western Port Bay during the 2010–11 year.

The following table shows discharge from each catchment in the Melbourne region.The discharge measured at each of the gauge locations used to calculate catchment outflows, plus the estimate of the ungauged discharge downstream of those gauges.

Discharge per catchment in the Melbourne region

Catchment

Flowgaugeno.

Flow gaugelocation

Gaugeddischarge (ML)

Estimatedungaugeddischarge (ML)

Totaldischarge (ML)

Ungaugedflows(% of total)

Bunyip

228209

Lang Lang River at Lang Lang

121,514

59,165

180,679

33

228213

Bunyip River at Iona

186,074

41,204

227,278

18

228228

Cardinia Creek at Cardinia

36,513

42,669

89,178

48

228217

Toomuc Creek at Pakenham

9,996

n/a

Downstream ungauged catchments

-

661,061

661,061

100

Bunyip discharge total

354,097

804,099

1,158,196

69

Yarra

229143

Yarra River at Chandler Highway

886,580

57,145

943,725

6

Yarra discharge total

886,580

57,145

943,725

6

Maribyrnong

230200

Maribyrnong River at Keilor

218,723

3,522

222,245

2

Maribyrnong discharge total

218,723

3,522

222,245

2

Werribee

231204

Werribee River at Werribee (Diversion Weir)

100,494

3,038

103,532

3

n/a

Downstream ungauged catchments

-

74,429

74,429

100

Werribee discharge total

100,494

77,467

177,961

44

Total discharge

1,559,894

942,233

2,502,127

38

Quantification Approach

Data Source

Melbourne Water Corporation: Waterworks database and Hydstra database for gauges in the Bunyip and Yarra catchments, Department of Sustainability and Environment: Hydstra database for gauges in the Maribyrnong and Werribee catchments.

Provided by

Melbourne Water Corporation and Department of Sustainability and Environment.

Method

Discharge from the Melbourne region to Port Phillip Bay and Western Port Bay was calculated using gauged streamflow data, plus the estimated ungauged flows in downstream subcatchments.

Gauged streamflow information is the sum of discharges during 2010–11 from the lowest stream gauges in the catchment that are above the tidal limit. Discharge was derived from a telemetered flow level gauge and the water level was converted to a volume using a stage–discharge relationship.

Ungauged streamflow was calculated using mean average flows (MAF) to provide a relationship between gauged streamflow data and flow magnitudes for adjacent or nearby catchment areas. Recorded flows were extrapolated to ungauged areas using the ratio of the MAF computation for the gauged area compared with the MAF computed for ungauged areas. The MAF values were based on the Sustainable Diversions Limit Project Department of Sustainability and Environment 2010a. These were updated in some catchments for the Central Region Sustainable Water Strategy (2006).

This is a change from the method used in the 2010 Account, which used measured discharges at the most downstream gauges in the region above the tidal limit to represent outflow. An estimate of surface discharge downstream of the gauges was not made.

Assumptions, Limitations, Caveats and Approximations

Gauged streamflow measurements are based on the the stage–discharge relationship at each gauge location. Stage–discharge relationships change over time and the accuracy measurement is dependent the location.

The portion of the catchment that is ungauged is 38%.

Conceptually the accuracy of the MAF in the ungauged portion of a catchment is likely to be low whereas the accuracy of the gauged outflows is likely to be high.

Uncertainty Information

The uncertainty estimate was not quantified.

Comparative year

This item corresponds to line item 14.2 Surface water flow out of region reported in the 2010 Account. In the 2011 Account, improved datasets were made available, which caused the 2009–10 value to be restated. The improved datasets were identified following a review of the method used to estimate discharge. The changes to the datasets used to estimate this line item are summarised as:

inclusion of estimates of ungauged discharge from catchment areas

exclusion of streamflow gauges 228203, 228356, 228363, 228365 and 229660 from the calculation of the gauged Bunyip River discharge

exclusion of streamflow gauge 229624 from the calculation of the gauged Yarra River discharge

exclusion of streamflow gauge 229643 from the calculation of the gauged Maribyrnong River discharge

exclusion of streamflow gauges 231107 and 231110 from the calculation of the gauged Werribee River discharge.

A restatement of the 2010 year volume was made. Increased accuracy in the measurement technique has resulted in a difference considered material. The restated comparative year volume is 795,254 ML and is summarised in the following table.

Restated comparative year information for line item River outflow from the region